aragonite

Aragonite is a carbonate mineral consisting of calcium
carbonate (CaCO3). It is formed under special conditions, generally
in caverns and hot springs. Present day mollusc shells are made of aragonite. It readily converts to calcite (another mineral form of calcium carbonates). It occurs as orthorhombic
system groups of needle-like crystals or massive deposits. Its appearance
is glassy white. Hardness 3.5–4, specific gravity 2.9.